I'm disappointed to see that no one ever answered this post. I have also noticed a significant difference between LiveStrong's and FatSecret's recommendations: 1600 on LS and 2300 on FS. I've been using LS for quite a while and setting it to lose 1.5 lbs a week really works. I'm very skeptical that setting FS for "slow weight loss" recommends 700 more calories.

The number is only an estimate, an approximation. No matter what number you start with, differences between individuals mean that most people who try to maintain a specific deficit for regular weight loss will have to be aware of whether they are meeting their goals and adjust their intake up or down as needed.